A disc drive typically includes a base to which various components of the disc drive are mounted. A top cover cooperates with the base to form a housing that defines an internal, sealed environment for the disc drive. The components include a spindle motor, which rotates one or more discs at a constant high speed, and an actuator assembly for writing information to and reading information from circular tracks on the discs. The actuator assembly includes a plurality of actuator arms extending towards the discs, with one or more flexures extending from each of the actuator arms. Mounted at the distal end of each of the flexures is a read/write head, which includes a fluid bearing slider enabling the head to fly in close proximity above the corresponding surface of the associated disc during operation of the disc drive. The fluid can be air or, alternatively, can be a different fluid such as, but not limited to, helium. When the disc drive is powered down, the heads may be moved to a landing zone at an innermost region of the discs where the fluid bearing sliders are allowed to land on the disc surface as the discs stop rotating. Alternatively, the actuator assembly may move (unload) the heads beyond the outer circumference of the discs so that the heads are supported away from the disc surface by a load/unload ramp when the drive is powered down.
Disc drives typically include a servo system for controlling the position of the heads during both seeking operations (moving from one track to another) and read/write operations where the head must precisely follow the circular track. One type of servo system is a dedicated servo system where one entire disc surface contains servo information written as dedicated tracks. The remaining disc surfaces within the drive are thus used to store data on dedicated data tracks. Another type of servo system, known as an embedded servo system, provides servo information on each of the disc surfaces embedded between data portions. Well known state estimator circuitry is used to estimate the position of the heads at such times that the heads are not located over the embedded servo information.
With both dedicated and embedded servo disc drives, servo information or “patterns” are typically recorded on the target disc by a servo-track writer assembly (“STW”) during the manufacture of the disc drive. One conventional STW records servo patterns on the discs following assembly of the disc drive. In this embodiment, the STW attaches directly to a disc drive and uses the drive's own read/write heads to record the requisite servo patterns to the mounted discs. An alternative method for servo pattern recording utilizes an “external STW” apparatus having dedicated servo recording heads for recording servo patterns onto one or more discs simultaneously prior to the assembly of such discs within a disc drive. An external STW that writes servo patterns to a single disc at a time is commonly referred to as a singledisc servo track writer (“SDW”), while an external STW that writes to multiple discs at a time is referred to as a multi-disc servo track writer (“MDW”).
It is crucial to provide a highly accurate positioning system with an STW to ensure accurate placement of the servo information on the discs. Specifically, an STW includes a positioning system for moving the actuator assembly and the attached heads across the disc surfaces during the servo writing procedure. The STW further includes a highly precise position detection system (often times incorporating a laser) for determining the position of the actuator assembly during the servo writing procedure. The position detection system provides correction signals to a motor within the positioning system to correct any errors in the position of the servo heads during operation of the STW.
In an effort to store more data onto existing or smaller-sized discs, the disc drive industry is continually attempting to increase the capacity of each disc or platter by increasing the track density (i.e., the number of tracks per millimeter). Increased track density requires more closely spaced, narrow tracks and therefore enhanced accuracy in the recording of servo-patterns onto the target disc surface. However, as the track density increases, it becomes increasingly likely that errors will be encountered during the servo writing process. For example, the servo writing head may experience resonance vibrations during operation, which alter the position of the head as the servo information is written. Such vibrations can lead to inaccurate servo information being written to the disc surface which, in turn, limits the ability of the disc drive to accurately position the data head over the desired data track during normal track following procedures (i.e., during normal read and write operations).
The resonance vibrations experienced by the head during the servo writing process are typically caused by the high-speed rotation of the discs within the STW. That is, the rotation of the discs within the STW (at speeds of up to 10,000 revolutions per minute or more) causes a great deal of air turbulence within the STW. This turbulence results from friction between the spinning disc surfaces and the air within the STW and represents a known phenomenon in the disc drive art. The air turbulence within a STW also impacts other components within the STW such as the actuator arms and the heads flying over the discs.
One proposed solution for reducing air turbulence while writing servo information to the discs is to partially fill the STW with helium gas during the servo writing process, thereby reducing the overall density of the gas within the disc drive. Specifically, reducing the density of the gas within the STW acts to reduce the frictional forces applied to the spinning discs, thereby reducing the drag-induced vibrations on the discs and the actuator assembly. However, a key disadvantage to this solution is that it is difficult to maintain desired helium concentrations within the STW due to the tendency of the helium gas to escape the confines of the STW during the servo writing procedure. Furthermore, because the servo writing heads must be loaded onto the discs and unloaded from the discs in an air environment (to prevent the heads from contacting the discs in a low-density helium environment), the helium gas must be flushed from the enclosed STW at the conclusion of each servo writing procedure. The difficulties associated with keeping the STW filled with helium are amplified in the case of external STWs which typically define a much larger volume, in relation to the volume of a disc drive, when the external STW is covered to provide a substantially closed environment for the helium gas. Thus, with respect to external STWs (e.g., an SDW or an MDW), each servo writing procedure requires large volumes of clean-room-quality helium gas in order to fill the relatively large interior volume of the external STW with a sufficient concentration of helium gas.
Accordingly there is a need for an improved external STW that can maintain desired concentrations of helium or other low-density gases in a cost-effective manner. The present invention provides a solution to this and other problems, and offers other advantages.